The invention relates to a semiconductor laser having a semiconductor body comprising a substrate of a first conductivity type and a layer structure disposed thereon having at least a first passive layer of the first conductivity type, a second passive layer of the second opposite conductivity type and an active layer located between the first and the second passive layer and further comprising a pn junction which can produce, at a sufficiently high current intensity in the forward direction, coherent electromagnetic radiation in a strip-shaped active region located within a resonant cavity, the first and second passive layers having a smaller refractive index for the radiation produced and a larger band gap than the active layer and the resonant cavity being formed by a periodical variation in the effective refractive index in the longitudinal direction and over at least part of the length of the active region. The active region is bounded by end surfaces substantially at right angles to the active region and an anti-reflection layer is provided on at least one of said end surfaces and is in direct contact therewith, the second passive layer and the substrate being electrically connected to connection conductors.
Such a semiconductor laser is described in the article by M. Yamaguchi et al in Electronics Letters, Vol. 20, No. 6, Mar. 15, 1984, p. 223-236.
Semiconductor lasers of different constructions are used in many fields. The resonant cavity can then be formed in different ways. In many cases, it is formed by two parallel extending mirror surfaces, for which use is mostly made of cleavage surfaces of the semiconductor crystal. By repeated reflections on these mirror surfaces, radiation modes known under the dseignation Fabry-Perot (FP) modes are produced.
According to another embodiment, the resonant cavity is obtained by periodical variation of the effective refractive index for the radiation produced over at least part of the length of the active region. Instead of reflection on mirror surfaces, reflection at a lattice (formed by the said periodical variation in the refractive index) is then used. Lasers in which this is the case are designated as lasers with distributed feedback. They exist in various constructions and are known under the designation "distributed feedback" (DFB) and "distributed bragg reflection" (DBR) lasers. In this Application, they will be designated as "DFB lasers" for the sake of simplicity.
DFB lasers have inter alia the advantage with respect to the aforementioned Fabry-Perot lasers that they can oscillate more readily in a single stable longitudinal oscillation mode ("Single Longitudinal Mode" or SLM mode) within a large temperature range and at a high output power. This is especially of importance in optical telecommunication applications because in the SLM mode the chromatic dispersion is a minimum so that the signal can be transported without interference over a larger distance through the optical fiber. Further, DFB lasers can be integrated comparatively readily in an electro-optical monolithic circuit.
However, since in general a DFB laser also has, at the ends of the active region, end surfaces at right angles to the active layer, Fabry-Perot oscillations can also occur between these surfaces so that in principle the DFB laser exhibits at least one FB mode besides at least one DFB mode having substantially the same amplification.
In order to suppress the Fabry-Perot mode, which is undesirable in DFB lasers, several measures have been suggested, as appears from the aforementioned article by Yamaguchi et al. The most suitable of these measures is that an anti-reflection layer is provided on one of the end faces. If this layer has a thickness corresponding to an optical path length equal to a quarter wavelength of the radiation produced, the reflection coefficient at this end face is reduced to a minimum value. If this value is sufficiently low, the occurrence of FB modes is avoided.
The said minimum value is for the amplitude reflection coefficient theoretically: EQU .delta.=(n.sub.1 n.sub.3 -n.sub.2.sup.2)/(n.sub.1 n.sub.3 +n.sub.2.sup.2) (1),
where
n.sub.1 =refractive index in the active layer, PA1 n.sub.2 =refractive index in the anti-reflection layer, and PA1 n.sub.3 =refractive index in the medium (air).
As anti-reflection layers, layers of aluminum oxide and of silicon nitride have been used. These layers are also used frequently in other applications of semiconductor technology and the technique of applying them is therefore well known. However, the use of these layers as anti-reflection layers on a laser has several disadvantages.
For example, aluminum oxide has proved to be less suitable because its refractive index (about 1.65) does not lead to a very low minimum value for the reflection coefficient at least with long-wave lasers (.lambda.=1.3 and 1.55 .mu.m) having a layer structure provided on a substrate of InP which are important for optical telecommunication. As a result, with higher current intensities Fabry-Perot modes may nevertheless be produced.
Silicon nitride has a refractive index which is more satisfactorily adapted to the application (about 1.85), it is true, but can be applied only by sputter techniques (for example plasma CVD), which is technologically less attractive, inter alia because the material is then deposited not only on the desired end face, but on all parts of the surface. This in contrast, for example, with application by vapor deposition.